RTN as chemoreceptor Flashcards
What did Haldane and Priestdley do?
Haldane & Priestly 1905 noted the ventilation approximately doubles for every 1.5mmHg rise in alveolar, and by extension arterial, PCO2. This hypercapnic ventilatory response is sensed predominantly at the level of the brainstem, since it is preserved even after denervation of the carotid body chemoreceptors. The identity of the central chemoreceptor (CCR) is uncertain, and there are two main schools of thought; A) the retrotrapeziod nucleus (RTN) is the predominant and specialised CCR, B) there is a ‘distributed chemoreceptor’ in the brainstem, with many different centres responsible to different degrees.
What did Loeskscke and Mitchell do?
Loeschcke & Mitchell demonstrated that the CCR was responsive to PCO2 by proxy of pH, since if pH was kept normal an increase in PCO2 failed to induce hyperventilation. CO2 is converted into carbonic acid and thus H+ ions by the action of carbonic anhydrase.
What did Pagliardini et al 2011 do?
- the lateral RTN/pFRG is the most sensitive site for induction of active expiration without a concomitant increase in inspiration.
- The medial RTN/pFRG in chemosensitivity but not in rhythmogenesis or production of active expiration
What is the RTN?
- The RTN is a collection of glutaminergic neurones which lie between the ventral medullary surface and the facial nucleus.
- Important work by Mulkey et al 2004 demonstrated that the RTN neurones had many of the properties that we would expect of a CCR:
i. They were activated by pH changes within a physiologically relevant range.
ii. Their ability to respond to changes in PCO2 in vivo persisted even after inhibition of the central pattern generator of respiratory rhythm or inhibition of the carotid bodies.
iii. The RTN neurones have extensive dendritic connections with the pontomedullary respiratory centres and with the ECM next to blood vessels on the ventral medullary surface. Thus the neurons are perfect for ‘tasting’ the PaCO2 and relaying this to the respiratory neurones.
What did Nattie et al 2002 and Akilesh et al 1997 do?
RTN neurones express neurokinin 1 receptors (NKR-1). Substance P is an agonist of the NKR, and it can be conjugated with the neurotoxin saporin to selectively destroy NKR expressing neurones in the RTN. Nattie et al 2002 used this technique and demonstrated that a 44% loss of RTN neurones reduced the CO2 ventilatory response by 30%. Akilesh et al 1997 used excitotoxin lesioning of 35% of the RTN neurones, with a concomitant loss 39% loss of the hypercapnic ventilatory response.
What did Gourine et al 2010 do?
RTN neurones characteristically express transcription factor Phox2b. Gourine et al 2010 fluorescently labelled these Phox2b neurones using a green fluorescent protein and an adenoviral vector. They were able to demonstrate a reversible depolarisation of all recorded RTN neurones in response to a decrease in pH.
What did Kumar et al 2015 do?
Kumar et al 2015 demonstrated that selective expression of the proton-activated receptor GPR4 in chemosensory neurons of the mouse RTN is required for CO2-stimulated breathing.
- GPR4 senses H+ with a pH-sensitive potassium channel called TASK-2.
- Genetic deletion of GPR4 disrupted acidosis-dependent activation of RTN neurons, increased apnea frequency, and blunted ventilatory responses to CO2. Reintroduction of GPR4 into RTN neurons restored ventilatory phenotype.
- Additional elimination of TASK-2, a pH-sensitive K+ channel expressed in RTN neurons, essentially abolished the ventilatory response to CO2.
What did Bayliss et al 2015 do?
Bayliss et al 2015 Despite widespread expression of acid-sensitive TASK-1 and TASK-3 channels at multiple levels within the brainstem, experiments in knockout mice provided no evidence for their involvement in CO2 regulation of breathing. However, alkaline activated TASK-2 channels are restricted to Phox2b-expressing chemoreceptor neurons of the RTN.
- genetic deletion of TASK-2 blunted RTN neuronal pH sensitivity in vitro, diminished the ventilatory response to CO2/H+ in vivo.
- Phox2b(27Ala/+) mouse genetic model of congenital central hypoventilation syndrome (CCHS) is characterized by reduced central respiratory chemosensitivity selective ablation of Phox2b-expressing RTN neurons was accompanied by a corresponding loss of TASK-2 expression.
- CA: However, a subpopulation of RTN neurons from TASK-2(-/-) mice retained their pH sensitivity, suggesting that other mechanisms for RTN neuronal pH sensitivity are yet to be identified.
Dubreuil V et al 2008 ?
Congenital central hypoventilation syndrome (CCHS) is caused by a genetic mutation of the Phox2b gene involving a polyalanine expansion. This results in hypoventilation or apnoea during sleep, and an absent response to hypercapnia. Thus it would appear that functioning Phox2b expressing neurones in the RTN are essential to normal central chemosensitivity, and that disruption of normal RTN function severely attenuates the normal respiratory response to hypercapnia.
What did Takakura et al 2008 do?
This was supported by Takakura et al 2008 who injected neurotoxin saporin-substance P (SSP-SAP) and found a dramatic rise in apnoeic threshold of PCO2 (suggestive of abolished CCR). Interestingly, normal phrenic nerve discharge (PDN) could be elicited by strong peripheral chemoreceptor stimulation in 8/12 rats.
Nattie & Li 2009?
Idea of a ‘distributed’ central chemosensitivity is supported by the evidence that in conscious states lesions of:
- Glutamatergic neurons in the RTN/pFRG,
- serotonergic neurons of the medullary raphe,
- locus ceruleus noradrenergic neurons or
- NK-1 receptor expressing cells in the ventral medulla
Serotonogernic neurones of medullary raphe?
Serotonergic (5-HT) neurones of the medullary raphe:
Many studies have demonstrated that these raphe neurones are chemosensitive in vitro, and they are well anatomically placed near cerebral blood vessels. Saporin conjugated to an antibody for serotonin reuptake inhibitor was used by Dias et al 2007 to kill 50% of the raphe neurones. This was associated with a 62% reduction in the hypercapnic response.
Noradrenergic neurones of locus ceruleus
These neurones also have chemosensitive properties in vitro. Biancardi et al 2007 bilaterally destroyed locus ceruleus neurones in vivo, and reported a concomitant 64% reduction in the respiratory response to CO2.
CA: Although these structures have been found to contain neurons that are excited by rising levels of PCO2/[H+] in vitro, whether these ‘chemoresponsive’ neurons are functional (in vivo) respiratory chemosensors is not known in most cases.
Medullary glial cells?
- It has recently emerged that glial cells of the ventral medulla may be sensitive to changes in PaCO2, and subsequently activate respiratory neurones through the release of gliotransmitters.
- Gourine et al 2005 blockade of ATP receptors in the ventral medullary chemoreceptive areas diminished the hypercapnic ventilatory response. In contrast, application of exogenous ATP stimulated breathing. Thus ATP seems to be an excitatory transmitter in the chemoreceptor mechanism.
- In 2010 the same group used anaesthetised, artificially ventilated cats, and demonstrated that medullary astrocytes responded to reduced pH with elevated intracellular Ca2+ and consequent release of ATP. This was intrinsic astrocyte chemosensitivity. Thus it was proposed that astrocytes of the ventral medulla are chemoreceptive and release ATP which acts via P2 receptors to stimulate other astrocytes and respiratory neurones. Astrocytes have many processes in contract with cerebral blood vessels, so they are well place to recognise hypercapnia.
Conclusion?
Whilst it appears that the RTN plays an integral role in vivo in sensing CO2 and modulating breathing, it is fair to suggest that other regions of the brainstem also play an important role. That said, a recent review by Guyenet et al 2008 argues that the RTN is the only area which has been shown to have appreciable effects on respiration in intact physiology. Injection of toxins into the brain can cause mechanical disruption and non-specific neuronal loss, so it is difficult to dissect out the role of specific neurones. The emerging evidence suggesting a role of glial cell is also persuading. Thus, the idea that the RTN is the sole CCR is perhaps a little too simplistic to be physiologically accurate.
